An elset (element set) is one snapshot of a satellite's orbit: a set of mean orbital elements valid at a stated epoch, packaged for a propagator such as SGP4. The TLE is its classic carrier. Fresh elsets are issued as new tracking data arrives — often several per day for low-orbiting objects.
Understanding Elset
Mean elements are not measurements
An elset is the output of a least-squares fit: an orbit-determination process takes days of radar and optical observations and finds the mean-element orbit that, when run through SGP4, best matches them. The elements therefore absorb the quirks of the theory — force-model simplifications end up baked into the fitted values, especially the drag term, which soaks up whatever the model cannot represent. Feed the same elements into a different, "better" propagator and the answer gets worse, not better. This is the most common mistake newcomers make with orbital data: treating catalogue elements as physical truth rather than as coefficients of a specific model.
Elset vs ephemeris vs state vector
| Product | What it is | Best for |
|---|---|---|
| Elset (TLE/OMM) | Mean elements + epoch, one orbit summary | Catalogue-scale tracking, pass prediction |
| State vector | Exact position + velocity at one instant | Precision work, initialising integrators |
| Ephemeris | Table of states over a time span | Conjunction screening, operator coordination |